ARTICLE pubs.acs.org/JAFC
Feasibility Study on Chemometric Discrimination of Roasted Arabica Coffees by Solvent Extraction and Fourier Transform Infrared Spectroscopy Niya Wang, Yucheng Fu, and Loong-Tak Lim* Department of Food Science, University of Guelph, Guelph, ON N1G 2W1, Canada
bS Supporting Information ABSTRACT: In this feasibility study, Fourier transform infrared (FTIR) spectroscopy and chemometric analysis were adopted to discriminate coffees from different geographical origins and of different roasting degrees. Roasted coffee grounds were extracted using two methods: (1) solvent alone (dichloromethane, ethyl acetate, hexane, acetone, ethanol, or acetic acid) and (2) coextraction using a mixture of equal volume of the solvent and water. Experiment results showed that the coextraction method resulted in cleaner extract and provided a greater amount of spectral information, which was important for sample discrimination. Principal component analysis of infrared spectra of ethyl acetate extracts for dark and medium roast coffees showed separated clusters according to their geographical origins and roast degrees. Classification models based on soft independent modeling of class analogy analysis were used to classify different coffee samples. Coffees from four different countries, which were roasted to dark, were 100% correctly classified when ethyl acetate was used as a solvent. The FTIR-chemometric technique developed here may serve as a rapid tool for discriminating geographical origin of roasted coffees. Future studies involving green coffee beans and the use of larger sample size are needed to further validate the robustness of this technique. KEYWORDS: solvent extraction, FTIR, coffee, discrimination, chemometrics
’ INTRODUCTION Coffee is one of the most popular beverages in the world due to its unique aroma, taste, and stimulating effects of caffeine. The quality of brewed coffee is affected by many parameters. Depending on the species (Arabica, Robusta, or Liberica) and method used to process the coffee cherries (dry vs wet), the overall quality and chemical composition of coffee bean can vary considerably. By and large, the Arabica coffees have more pronounced and finer flavor profiles that are considered better quality and, accordingly, command a higher price than the Robusta and Liberica coffees.1 The composition of the soil and its fertilization, the altitude and weather of the plantation, and the final cultivation and drying methods used will all affect the green bean quality.2 Roasting, the final processing step before grinding and brewing, ultimately determines the organoleptic properties of the coffee beverage. During the roasting process, the reactions that occur in the coffee bean are complex and strongly dependent on the timetemperature profile used.4,30 Grading of whole coffee beans (green or roasted) is relatively easy as compared to ground coffee due to the presence of visual clues in the former (size, shape, defect, etc.). By contrast, these indicators are absent for ground coffees; therefore, sample discrimination can be difficult. Often time, sensory evaluation and cupping are needed.5 Analytical methods have been successfully used for compositional analysis of coffee, including mineral contents,5,6 volatile compounds,7 chlorogenic acids,8 fatty acids,9 and amino acid enantiomers.10 Fourier transform infrared (FTIR) spectroscopy is a rapid and nondestructive technique that has been used for investigating covalent bond vibration in coffee. This method has been used to r 2011 American Chemical Society
determine the caffeine content in roasted coffee,11,12 to discriminate coffee varieties,11,13,14 and to detect adulteration in instant coffees.15 Because of the complexity of FTIR spectral data, chemometric analysis [e.g., principal component analysis (PCA) and soft independent modeling of class analogy (SIMCA)] is often used to reduce the dimensionality of spectral data to aid the extraction of useful information, identification of natural data trends, and classification of unknown samples.16 Chemometric analysis has been successfully applied to analyze FTIR spectral data of coffee, for instance in the chemical discrimination of Arabica and Robusta coffees,17 quality control and authentication of instant coffees,18 and adulteration detection of freeze-dried instant coffees.15 In this study, we employed attenuated total reflectance (ATR)FTIR to analyze coffee extracts prepared using six organic solvents (dichloromethane, ethyl acetate, hexane, acetone, ethanol, and acetic acid). Our objective was to investigate the feasibility of using infrared spectra of these extracts, in conjunction with PCA and SIMCA, to discriminate four Arabica ground coffees from different origins (Colombia, Costa Rica, Ethiopia, and Kenya) that had been roasted to two roast degrees (medium or dark).
’ MATERIALS AND METHODS Chemicals. Hexane was purchased from Sigma-Aldrich Ltd. (St. Louis, MO). Dichloromethane, ethyl acetate, acetone, and acetic Received: December 29, 2010 Accepted: March 7, 2011 Revised: March 3, 2011 Published: March 07, 2011 3220
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Table 1. L* Value of Roasted Ground Arabica Coffee Beans roast degree dark
medium
Figure 1. Air temperature (in roast chamber) profiles of the fluidized bed hot air coffee roaster. acid were purchased from Fisher Scientific (Ottawa, Canada). Ethanol was purchased from Greenfield Ethanol Inc. (Brampton, Canada). Coffee Beans and Roasting Conditions. Wet-processed green coffee beans (Arabica variety) from Colombia, Costa Rica, Kenya, and Ethiopia were purchased from Green Beanery (Toronto, Canada). Green coffee beans (45 g) were roasted in a fluidized bed hot air roaster (Fresh Roast SR 500, Fresh Beans Inc., Park City, UT). Two isothermal roasting programs were used for preparing dark and medium roast coffees (Figure 1). The roasted beans were stored in hermetic glass bottles in the dark at 15 °C before grinding.
Degree of Roast as Determined by Color Measurements. Roasted coffee beans were milled into powder using an electric burr grinder (Bodum Antigua, Bodum, Inc., Copenhagen, Denmark) at the medium grind setting. The color of the ground coffee was measured in the L*, a*, b* system using a Konica Minolta CM-3500d spectrophotometer (Konica Minolta Sensing, Inc., Osaka, Japan) in the reflectance mode. Before analysis, the instrument was calibrated on a white standard tile. Measurements were taken in triplicate. Solvent Extraction of Ground Coffee. After grinding, coffee grounds were extracted with dichloromethane, ethyl acetate, hexane, acetone, ethanol, or acetic acid, following two extraction procedures. In the first procedure (method #1), 0.2500 g of ground coffee was accurately weighed into a glass vial, and 1 mL of deionized water was added to wet the sample. The glass vial was shaken for 1 min with an IKA-VIBRAX-VXR vibrator (Janke & Kunkel, Inc., Staufen, Germany) at the 200 dial setting; 1 mL of organic solvent was added, and the mixture was shaken for an additional 5 min. The organic phase was then transferred to another vial and allowed to rest for 10 min before ATRFTIR analysis. In the second procedure (method #2), a similar procedure was used except that water was not added prior to solvent extraction. All extractions were performed in triplicate. ATR-FTIR Analysis. The coffee extract was scanned using an FTIR spectrometer (IR Prestige-21; Shimadzu Corp., Tokyo, Japan) equipped with a deuterated triglycine sulfate detector and a KBr beam splitter. A MIRacle ATR accessory equipped with a diamond crystal (Pike Technologies, Madison, WI) was used for sampling. The background spectrum was collected using an empty ATR cell. Before scanning, a drop of extract (6 μL) was placed onto the ATR crystal, and the solvent was allowed to evaporate. The time required for the solvent to evaporate was determined by monitoring the spectrum until the solvent bands were no longer detectable. The time taken for this to occur was noted and applied to all extracts prepared using a given solvent. By removing the background absorbance interference from the solvent, the sensitivity
coffee bean sample
lightness (L*)
Colombian
19.83 ( 0.05
Costa Rican
19.61 ( 0.18
Ethiopian
19.46 ( 0.21
Kenyan
19.72 ( 0.06
Colombian
25.21 ( 0.16
Costa Rican
25.35 ( 0.29
Ethiopian
25.64 ( 0.06
Kenyan
25.28 ( 0.09
of the chemometric analysis was improved considerably. To collect the IR spectrum, samples were scanned from 600 to 4000 cm1 at 4 cm1 resolution, and 20 scans were averaged to give the final spectrum. For each extract, three FTIR spectra replicates were scanned. Between samples, the ATR crystal was carefully cleaned with 95% (v/v) aqueous ethanol solution and dried with lint-free tissue paper. The spectral baseline was examined to ensure that no residue from the previous sample was retained on the crystal. All spectra were recorded at room temperature (23 ( 0.5 °C). Data Analysis. Statistical comparison of color values of ground coffee samples was conducted based on Tukey pairwise comparisons using R software (www.r-project.org). For chemometric analysis, FTIR spectra were exported as ASCII format, organized in Excel spreadsheets, and then analyzed using Pirouette v.4.0 software (Woodinville, WA). During PCA, second derivative and mean-center were applied to FTIR spectra to reduce baseline variation and enhance spectral features. Nine spectra (three extracts for each coffee and three replicate spectra for each extract) for each coffee were divided into two groups: Six spectra from the first two extracts were used to calibrate the SIMCA model, while the remaining three spectra from the third extract were used for validation to evaluate the prediction accuracy of the calibrated SIMCA model. The optimum number of PCs in each class was selected on the basis of the lowest number of PCs giving minimum value of variance.
’ RESULTS AND DISCUSSION Color Analysis. Ground coffee samples from different geographical regions could not be distinguished readily by visual inspection. The L* (lightness) values of ground coffee beans from different geographic regions (Colombia, Costa Rica, Ethiopia, and Kenya) were similar among medium roast or dark roast samples (Table 1). Tukey pairwise comparison analysis confirmed that differences in L* values were not significant between ground samples for dark or medium roasted beans, implying that samples from the same degree of roast exhibited the same lightness. ATR-FTIR Analysis. Selected FTIR spectra of solvent extracts obtained by methods #1 (with water) and #2 (no water) are shown in Figure 2. The 3100 to 2750 cm1 region in the majority of spectra (except acetic acid, acetone, and ethanol extracts obtained with extraction method #1) were typical for the fatty acid moiety of lipids due to asymmetrical CH stretching (2920 cm1), symmetrical CH stretching (2850 cm1), and methylene asymmetrical stretching (weak shoulder at 2954 cm1).19 In the presence of water, the absorbance around 36763028 cm1 for acetic acid, acetone, and ethanol extracts can be attributed to the OH stretching band. The 1800 800 cm1 region contained absorbance bands due to CdO (ester, aldehydes, and ketones) stretching, CH (methylene) bending (scissoring), CO (esters and alcohol), and CH2 3221
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Figure 2. FTIR spectra of coffee extracts obtained with hexane, dichloromethane, ethyl acetate, acetone, ethanol, or acetic acid using method #1 (with water) and method #2 (no water).
stretching/bending.19 These regions contained fingerprint information that may be important for discriminating coffee samples from different origins. Spectra from method #1 were relatively more complex than those from method #2, especially when dichloromethane and ethyl acetate were used as solvents. For instance, the dichloromethane extracts prepared from method #1 had many additional peaks that were absent for those prepared from method #2, including 1487 (CdC, CH deformation), 1398 (CH3 symmetric deformation), 1323 (symmetric vibrations of COO groups), and 1284 cm1 (amide III band components of proteins).20 In terms of band shape and intensity, different spectral features were observed in the 17201203 and 1064 940 cm1 regions. With method #1, water-induced swelling of the coffee particles might have facilitated the extraction of additional compounds. A similar enhancement in spectral features was observed for ethyl acetate coffee extracts. For the hexane and acetic acid extracts, minimal spectral differences were observed between methods #1 and #2. The IR spectra of the hexane extracts were similar to lipid,21 indicating that lipids may be the main components extracted. Overall absorbance values were considerably stronger for the acetone and ethanol extracts probably due to the contribution from water present in the extracts. The spectra of acetic acid extracts and pure acetic acid were similar (data not shown), indicating that acetic acid is not an effective solvent for coffee extraction. On the basis of the evaporation time data and FTIR spectral features observed, dichloromethane, hexane, ethyl
acetate, and acetone extracts obtained via method #1 were selected for subsequent analyses. PCA Analysis of Solvent Extracts of Coffee Beans. FTIR spectra of the organic solvent extracts (method #1) are highly complex. Although variances between spectra exist, the differences are subtle, and data interpretation was difficult (data not shown; see Figure S1 in the Supporting Information). To extract relevant information from the data, PCA was employed to reduce the dimensionality of the IR spectra and facilitate the visualization of the inherent structure of the data set (Figures 3 and 4). For the medium roast samples, FTIR data for dichloromethane and ethyl acetate extracts appeared as separated clusters in PCA score plots, which corresponded to the four countries of origin (Figure 3, row A); however, cluster patterns were less discernible for hexane and acetone extracts. For the dark roast samples, the PCA score plots showed clear distinctive groupings corresponding to the four countries of origin (Figure 4, row A). Overall, separation distances between clusters were greater for the dark roast samples than for the medium roast counterparts, implying that the IR-active components that were distinctive to the bean origin tended to develop when the beans were roasted to a darker degree. It is well-known that the aroma characteristic of coffee is strongly dependent on the timetemperature profile applied during the roasting process.30 The greater cluster separation for dark roast samples observed in the current study may be due to a larger number of country-specific aroma compounds produced in the dark roasted coffee.22 3222
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Figure 3. PCA of FTIR data for hexane, dichloromethane, ethyl acetate, and acetone extracts of medium roast coffee. Row A: Two factor score plots. Row B: Loading plots of PC1. Row C: Corresponding FTIR raw spectra.
The different clustering behaviors observed for extracts prepared from different solvents could be attributed to the different polarities of the solvents used. The polarity indices for hexane, dichloromethane, ethyl acetate, and acetone are 0.1, 3.1, 4.4, and 5.1, respectively.22 Thus, hexane is nonpolar and extracts only nonpolar compounds from the coffee. On the other hand, acetone is relatively more polar and tends to extract polar compounds. For dichloromethane and ethyl acetate, both polar and nonpolar compounds are extracted. The polarity effect can be observed in the original spectra (Figures 3 and 4, row C). The spectral region from 36763028 cm1 is mainly due to the OH stretching band from water. As shown, the absorbance intensity in this region progressively became stronger for hexane, dichloromethane, ethyl acetate, and acetone in ascending order. This result is consistent with the polarity for these solvents. To further investigate regions of spectra that contribute to the variance of samples, the loading plots for a corresponding PC were inspected. Here, we focused on PC1 since it explained the maximum variance existing in the data set (Figures 3 and 4, row B). The percent variance accounted by PC1 was also indicated on each loading plot. Regions of each spectrum with a relatively large loading score (>0.1) were highlighted as red dotted lines. As shown, the loading plots for hexane extracts were markedly different than those of the other three solvent extracts, due to the nonpolar nature of hexane. The loading plots of hexane extracts for medium and dark roasts were similar, except that absorbance at region 17411726 cm1, which is due to CdO stretching band mode of fatty acid esters, was higher and wider in the medium roast as compared with the dark roast coffee.23
For dichloromethane extracts, the most prominent difference in loading plots for dark and medium roast coffees was in the region of 29202850 cm1, which can be attributed to CH2 asymmetrical stretching vibrations of hydrocarbon methyl groups.24 The medium roast coffees exhibited significant loading score around this region but negligible for dark roast coffees. A similar trend was observed for the region around 1741 1678 cm1 The minimal changes observed for these spectral regions for the dark roast samples could be caused by a decrease in protein and lipids due to the Maillard reaction and pyrolytic cleavage, respectively.25,26 For ethyl acetate extracts, loading plots for medium and dark roast coffees were comparable, indicating that the compounds extracted by ethyl acetate from medium and dark roast coffees were similar, although subtle differences did exist. The main regions that contribute to the differences between samples are 17431741, 16471643, and 1697 cm1. The band at 1697 cm1 is due to isolated carbonyl stretching of CdO bonds, and the band at 1647 cm1 is due to conjugated carbonyl stretching of CdO bonds of caffeine compounds.27 Garrigues et al.12 and Ohnsmann et al.13 also utilized absorbance at 1659 and 1704 cm1 to determine the caffeine content in coffee and tea, respectively. In these cited studies, the CdO bands investigated shifted to higher frequencies due to the different solvent used (i.e., chloroform). On the basis of this information, it is hypothesized that the separated clusters observed were partly caused by the different caffeine contents of among the various coffee samples. Other important vibration bands that contributed to the separated clusters for dichromethane extracts were at 1705 (CdO stretching vibrations of ketones), 1655 (CdO stretching of caffeine 3223
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Figure 4. PCA of FTIR data for hexane, dichloromethane, ethyl acetate, and acetone extracts of dark roast coffee. Row A: Two factor score plots. Row B: Loading plots of PC1. Row C: Corresponding FTIR raw spectra.
compounds), 1599 (NH group), and 1548 cm1 (NH bending of peptide groups). These bands were also detected in ethyl acetate and acetone extracts with some shifts (1701, 1651, 1604, and 1552 cm1 for ethyl acetate; 1699, 1647, 1599, and 1558 cm1 for acetone).28,29 For hexane extracts, the most prominent spectral difference between the medium and dark roast coffees is that the latter showed a stronger overall absorbance, implying that more lipids (16001700 cm1) and fatty acid esters (17001800 cm1) were being extracted from the dark roast coffee. PCA Analysis for Coffees According to Degree of Roast. Roasting results in many physical changes and chemical reactions in the coffee beans. Depending on the extent of the roast, which is timetemperature dependent, the quality and sensory properties of the resulting coffees can vary considerably. Medium roast coffee has a more full-bodied flavor, a balance of taste and aroma, and carries citrus taste. In comparison, dark roast coffee has a heavier sweet taste, with a lingering aftertaste of chocolate.3,30 The FTIR spectra of dichloromethane and ethyl acetate extracts were analyzed for dark and medium roast coffees. The twocomponent score plots for these extracts show well-separated clusters corresponding to dark (right clusters) and medium (left clusters) roast samples for each coffee variety (Figure 5 for ethyl acetate extract; see the Supporting Information for dichloromethane extract). The loading plots for dichloromethane extracts showed that all coffee samples, except the Columbian coffee, exhibited strong loading scores at 2920, 2850, and 1743 cm1 due to CH2 asymmetrical stretching of methyl groups, CH symmetrical stretching of methyl groups, and CdO stretching of aliphatic esters.21,31 For the Colombian coffee, the bands that correspond to significant loading scores at 1550, 1510, and 1481 cm1 can be attributed to NH
bending of peptide groups, CdN stretching of amino groups, and benzene absorption bands, respectively.29,32,33 For ethyl acetate extracts, the loading plots (Figure 5, row B) revealed that spectral regions that contributed to cluster separation were mainly at 28502920 cm1 due to CH2 asymmetrical stretching and CH symmetrical stretching of methyl groups21 as well as 16501750 cm1 due to CdO stretching vibrations and CdN stretching.34 For coffee, this region has been assigned to a number of important compounds, including aromatic acids (17001680 cm1), aliphatic acids (17141705 cm1), ketones (17251705 cm1), aldehydes (17391724 cm1), and aliphatic esters (17551740 cm1).3537 Absorbance in the 28502920 cm1 region was mainly due to lipids.21 Overall, roasting coffee from a medium to a dark degree causes increases in esters/lactones (1788 cm1), aldehydes/ketones (17391722 cm1), ketones (17251705 cm1), aromatic acids (17001680 cm1), and aliphatic acids (17141705 cm1) but a decrease in caffeine content (17001692 and 1647 1641 cm1).20,30,31 Others have also observed decreases in the amount of lipids (around 1736, 1740, 1745, and 1750 cm1), polysaccharides and hemicelluloses (1739 cm1), esters (1751 1740 cm1), and lipids/proteins (29352847 cm1).20,30,31 SIMCA Analysis. Following the successful application of PCA techniques to discriminate selected coffee samples according to their geographical origin and degree of roast, SIMCA classification was employed to predict the origin and degree of roast for unknown samples. Table 2 shows the results of the prediction performance for coffee from different origins based on SIMCA models at the 5% significance level. Except for the dichloromethane extract for the 3224
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Figure 5. PCA of FTIR data for ethyl acetate extracts of coffee (from the same origin) with two degrees of roast. Row A: Two factor score plots. Row B: Loading plots of PC1. Row C: Corresponding FTIR raw spectra.
Table 2. SIMCA Classification Results for Coffees from Different Geographic Origins
Table 3. SIMCA Classification Results for Coffees According to Degree of Roast
country of origin correct classification (%) solvents dichloromethane ethyl acetate
roast degree Colombia
Costa Rica
Ethiopia
Kenya
roast degree correct classification (%) solvents
origin
dark
medium
dark
100
100
100
100
Colombia
100
100
medium
100
100
33
100
Costa Rica
100
100
dark medium
100 100
100 100
100 100
100 100
Ethiopia Kenya
100 100
100 100
Ethiopian medium roast sample, all other samples were correctly assigned to the country of origin during model validation. Similar validation results were obtained for the prediction degree of roast within each coffee (Table 3). Overall, ethyl acetate is a more optimal solvent for the discrimination of coffee origins and roasting degrees. Ethyl acetate is also a common solvent used for decaffeinating coffee and tea leaves.38,39 In summary, the solvent extraction and chemometric analysis methodologies presented in this study may be useful for the coffee industry as a rapid and reasonably accurate tool to classify roasted coffee according to origin and degree of roast. Future investigations involving more coffee varieties and bigger sample size are necessary to further improve the model robustness. By correlating the chemometric results with sensory data, potentially the methods may be useful for routine quality evaluation, which complement sensorial and cupping procedures.
dichloromethane
ethyl acetate
Colombia
100
67
Costa Rica
100
100
Ethiopia
100
100
Kenya
100
100
’ ASSOCIATED CONTENT
bS
Supporting Information. Typical FTIR spectra of dichloromethane extracts for dark roast coffee beans from various regions and PCA and spectral data of dichloromethane extracts for dark and medium roast coffees. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ1-519-824-4120, ext. 56586. Fax: þ1-519-824-6631. E-mail:
[email protected]. 3225
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Journal of Agricultural and Food Chemistry Funding Sources
This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Mother Parkers Tea and Coffee Inc.
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